A biogeographical approach to plant invasions: the importance of studying exotics in their introduced and native range

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Abstract
Summary: Most theory and empirical research on exotic invasions is based on the assumption that problematic exotics are much more abundant in the regions where they invade than in the regions where they are native. However, the overwhelming majority of studies on exotic plants have been conducted solely within the introduced range. With few exceptions, ecologists know surprisingly little about the abundance, interaction strengths and ecosystems impacts of even the best‐studied exotics in their native range. We argue that taking a biogeographical approach is key to understanding exotic plant invasions. On a descriptive level, unambiguous quantification of distributions and abundances of exotics in native and introduced ranges are crucial. Experiments conducted at a biogeographical scale are also necessary to elucidate the mechanisms that enable highly successful exotics to occur at substantially higher abundance in their introduced vs. native communities. We summarize the leading hypotheses for exotic plant success. We assert that tests of these major hypotheses for invasions (the natural enemies, evolution of invasiveness, empty niche and novel weapons hypotheses) require comparative biogeographical approaches. In addition to focusing on comparative work in the native and introduced range, we also suggest other approaches that could yield important insight into processes that influence exotic success. Increased understanding of invasions has the potential to provide unique insight into fundamental ecological theory, including that on individualistic‐holistic structure, the role of trophic interactions in population regulation, and the importance of co‐evolution in communities.